ISLAMABAD (AP) — A Pakistani cricketer-turned-politician on Sunday called on thousands of anti-government protesters to stop paying taxes and practice civil disobedience until Prime Minister Nawaz Sharif steps down, raising fears of instability in the nuclear-armed U.S. ally.

Imran Khan, who heads Parliament’s third largest bloc, made the announcement at a rally in the capital calling for Sharif to step down over alleged voting fraud in the May 2013 election, the first democratic transfer of power in a country with a long history of military dictatorships.

Khan also warned that his supporters would take over Parliament if Sharif does not resign within two days.

“We decide today that we will not pay taxes to his illegitimate government, we will not pay electricity bills, gas bills,” Khan said to a charged crowd estimated at 10,000 to 15,000 people. “I urge all the traders to stop paying taxes.”

Khan and cleric Tahir-ul-Qadri, a dual Canadian citizen with a wide following who spends most of his time abroad, have mounted twin protests that have brought thousands of people into the streets in Islamabad. They accuse Sharif of rigging the election that brought him to power.

Finance Minister and Sharif ally Ishaq Dar called Khan’s move unconstitutional, and Information Minister Pervaiz Rashid called the demand a “joke.” But both ministers told SAMAA TV that their government was ready to negotiate with Khan over his demands for electoral reforms within the constitutional framework.

Pakistani Interior Minister Nisar Ali Khan said the government was setting up two separate committees of party leaders to open negotiations with Khan and Qadri. “We are ready to accept all of your constitutional and legal demands,” he said.

While the crowds have fallen well short of the million marchers that both men promised, their presence and the heightened security measures have virtually shut down business in the capital.

The rallies have nevertheless remained festive, with families picnicking and men and women dancing to drums and national songs.

Police estimate that the crowds in both sit-ins have gradually dwindled since they arrived in the capital late Friday. Both rallies began as caravans of vehicles setting out from the eastern city of Lahore. Police official Nasir Shah estimated that there were currently around 25,000 to 30,000 people in both rallies.